Continuously cultured MCF-7 human breast cancer cells have been recently reported to be responsive to both androgens and estrogens. Studies in our laboratory suggest that this cell line is capable of the intracellular synthesis of biologically active androgen and estrogen from steroid precursor. If true, these phenomena could have significant implications in breast cancer therapy. The objectives of this proposal are to: 1) develop a convenient aromatase assay for use in cell culture experiments, 2) to investigate the relative activities, and potential hormonal regulation of the terminal steps of estrogen and dihydrotestosterone biosynthesis, 3) determine the influence of these sex steroids upon cellular levels of androgen and estrogen receptors and cell growth. The studies will involve the in vitro and in vivo determinations of aromatase and 5alpha-reductase enzyme activities, measurements of both cytoplasmic and nuclear levels of androgen and estrogen receptors, and determinations of thymidine incorporation, protein synthetic, and cell replication rates under a variety of experimental conditions. Investigations into the possibility of sex steroid regulation of human breast cancer, as well as further studies of the relative importance of the recently described biosynthetic potential of these steroids in some breast tumors, has been hampered by the lack of suitable experimental human breast cancer model systems. MCF-7 cells may represent such a model. It is anticipated that the results of this study will provide additional insights into the hormonal regulation of human breast cancers, which may eventually lead to the development of improved therapies.